TConnolly
Lifetime Supporting Member
I’m just curious, has anyone ever implemented a PID control for an evaporative cooling tower system?
I’ve never bothered to apply a PID to one but use a 4 state control based on water temperature deviation from the setpoint and duration of the deviation.
State 0: Tower fans off, tower feed pump off.
State 1: Tower feed pump is on
State 2: Tower feed pump is on, one tower fan is on
State 3: Tower feed pump is on, both tower fans are on.
From State = 0
At setpoint + 1 a timer starts. If it ever times out, the state increments.
At setpoint + 2 a shorter timer preset is used.
As setpoint +3 the state unconditionally increments.
From State = 1
While PV remains at SP+1 a long timer counts. If it times out, state increments.
While PV is at SP + 2 a shorter timer counts. It if times out – state increments.
and so on….. you get the picture of how it works (states are decremented in reverse)
As you can tell, there are three discretes, a feed pump and two fans. That’s it. No VFD on the feed pump or on the fans, only soft starters.
Typically on a cooling tower precise water temperature control is not necessary, but rather just staying within a band. The 4 state control works quite well.
However, I’m right in the middle of building yet another cooling tower control system, so I thought I would pick everyone’s brains.
I’ve never bothered to apply a PID to one but use a 4 state control based on water temperature deviation from the setpoint and duration of the deviation.
State 0: Tower fans off, tower feed pump off.
State 1: Tower feed pump is on
State 2: Tower feed pump is on, one tower fan is on
State 3: Tower feed pump is on, both tower fans are on.
From State = 0
At setpoint + 1 a timer starts. If it ever times out, the state increments.
At setpoint + 2 a shorter timer preset is used.
As setpoint +3 the state unconditionally increments.
From State = 1
While PV remains at SP+1 a long timer counts. If it times out, state increments.
While PV is at SP + 2 a shorter timer counts. It if times out – state increments.
and so on….. you get the picture of how it works (states are decremented in reverse)
As you can tell, there are three discretes, a feed pump and two fans. That’s it. No VFD on the feed pump or on the fans, only soft starters.
Typically on a cooling tower precise water temperature control is not necessary, but rather just staying within a band. The 4 state control works quite well.
However, I’m right in the middle of building yet another cooling tower control system, so I thought I would pick everyone’s brains.